It wasn't so much the stealth aspect that hurt MMs. Back during the period they are talking about you couldn't take pets into missions with you, you had to summon them inside then upgrade them. They also had MUCH longer recharge times and endurance costs then. So the ambush comes, overwhelms you and you die, you go to hospital and reenter the mission but they immediately target the MM again and head straight for them at the entrance, the chances are they kill you before you have finished summoning and upgarding. Next attempt (after waiting up to 2 minutes for pets to recharge) you enter and they are already waiting for you at the entrance and you have no time to summon pets before being killed again... and so on. Other ATs were at least back up at full strength going back into the missions, where MMs were at their weakest, which is why people said ambushes like that penalized MMs the most.